1. Field of the Invention
The present invention is in the field of incandescent lamps. More particularly, the present invention is directed to incandescent lamps which have two envelopes so as to reduce the exterior surface temperature of the lamp when it is in operation. Still more specifically, the present invention is directed to miniature and subminiature incandescent lamps that operate on low voltage.
2. Brief Description of the Prior Art
Incandescent lamps which have two envelopes are known in the art, and are described for example in U.S. Pat. Nos. 344,343; 512,464; 1,145,837; 1,194,643; 1,858,712; 2,010,772 and 2,177,755. Similar or related structures, such as metal vapor discharge lamps and and two-electrodespark gap apparatus having two envelopes or the like are described in U.S. Pat. Nos. 5,600,198; 5,453,655; 4,198,586; 3,906,273; 3,832,589 and 2,735,954.
U.S. Pat. No. 5,686,786 discloses an incandescent lamp that is suitable for use in conjunction with night vision equipment when the specialized glass envelope of the lamp is not transparent to light of certain wave length. The general structure described in this patent is also well suited for the manufacture of sub-miniature and miniature lamps without necessarily having the specialized glass envelope required for compatibility with night vision equipment.
As is known in the art, and stated in one more of the above-cited prior art patents (e.g. U.S. Pat. Nos. 344,343 and 1,145,837) the use of a second glass envelope enclosing the first envelope that contains the incandescent filament results, generally speaking, in lower temperature of the exterior surface of the second envelope than what the temperature of the exterior surface of the first envelope would be without further thermal insulation provided by the second envelope. In other words, as is known in the art, the second envelope results in lamp that is cooler on the outside than a comparable lamp would be without the second envelope.
Providing relatively cool exterior surface to incandescent lamps has particular importance in certain applications, such as in certain types of flashlights, lights used in hazardous situations or environments, in portable lanterns and in area spot and floodlights. In fact, there are Underwriter Laboratories standards for incandescent lamps used in certain applications where the standards require the outside surface temperature of the lamp not to exceded 160xc2x0 C. (the permissible maximum under the standard). In order to meet this Underwriter Laboratories standard the power output of certain incandescent lamps must be limited beyond that which would be otherwise desirable, because, as it is well understood in the art, greater power output results in brighter lamp, but also tends to increase the surface temperature of the lamp. The present invention is designed to accommodate the need for lamps with surface temperatures that are relatively cool (in case of miniature and sub-miniature lamps do not exceed 160xc2x0) and provides an incandescent lamp having two envelopes (resulting in said lower exterior surface temperatures) which are easily and relatively inexpensively manufactured using existing prior art lamps.
It is an object of the present invention to provide an incandescent lamp that has relatively cool exterior surface temperature.
It is another object of the present invention to provide an incandescent lamp construction that is well suited for the manufacture of miniature and subminiature lamps which have relatively cool exterior surface temperature that does not exceed 160xc2x0 C.
It is still another object of the present invention to provide an incandescent lamp that meets the foregoing objects and is readily manufactured from existing fully assembled incandescent lamps of a certain construction, generally described in U.S. Pat. No. 5,686,786.
The foregoing and other objects and advantages are attained by an incandescent lamp that includes a filament enclosed in a first glass envelope and supported by a pair of metal contact pins. A first glass bead seals the envelope and has a pair of openings through which the metal contact pins extend and wherein they are hermetically sealed. The first envelope, the filament, the supporting metal contact pins and the first glass bead together form a prior art incandescent lamp similar to the type disclosed in U.S. Pat. No. 5,856,786; that is these parts when assembled would form the aforesaid prior art lamp if they were not made to be part of the overall structure of the present invention. A second glass bead of low melting solder glass is larger than the first bead and is sealed below the first bead and extends circumferentially beyond the perimeter of the first bead, and a third bead is disposed below the second bead also in sealing relationship with the second bead. The metal contact pins extend through the second and third glass beads to serve as electric contacts when the lamp is to be energized. The outside perimeter of the third bead is substantially co-extensive with that of the second bead. A second glass envelope is sealed to the outside perimeter of the second bead and is also in abutting relationship with the third bead. The interior of the first envelope is evacuated or contains an inert gas of the type normally used in incandescent lamps. The interior hermetically sealed space between the first and second envelopes is also evacuated, or filled with an inert gas, but is preferably filled with nitrogen gas. The resulting miniature and sub-miniature lamps have operating exterior surface temperatures which do not exceed 160xc2x0 C.
The features of the present invention can be best understood together with further objects and advantages by reference to the following description, taken in connection with the accompanying drawings, wherein like numerals indicate like parts.